Photomultiplier tubes long have been used where changing light intensities are to be monitored. Sometimes the wide dynamic ranges of light flux can damage the tubes because of their high gain created self-destructive currents. Over the years a number of circuits have been developed for protecting photomultiplier tubes but for one reason or another they have disadvantages and limitations that make them unreliable or otherwise unacceptable.
The attenuation of incident optical light flux and the reduction of the dynode voltage are two methods most commonly used (alone and in combination) to accommodate the high-light-flux, large-anode-current region of a typical large dynamic range requirement characteristic of undersea optical receivers. These receivers have such a large dynamic range requirement to enable their functioning linearly from the sea surface in full daylight to great depths in partial moonlight. The dynamic range requirement spans approximately twelve orders of magnitude.
The conventional methods of protecting a photomultiplier tube operating over the wide dynamic range suffer from two shortcomings or limitations, namely, when optical attenuators are used they reduce the desired, often small optical signal (undesirable) as well as reducing the large ambient background light (desirable). Reduction of the dynode voltage is limited by the loss of anode collection efficiency and by the insufficient dynode bleeder (standby) currents at high collection current operation. If smaller dynode resistors are used to avoid this problem, the resultant high-wattage dissipation must be accepted and a much larger current capability and wattage from a variable voltage supply must be provided. Concommitant problems of space and power dissipation in a small confined package have deleterious effects on the optical receiver's linearity, stability and life.
Thus, there is a continuing need in the state-of-the-art for an apparatus and method for assuring a wide linear dynamic range capability for a photomultiplier tube that does not compromise its operational characteristics nor impose unreasonable power and packaging burdens.